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Microprocessor system architectures – IA32
tasks
Jakub Yaghob
Using tasks in OS
1 thread = 1 task 1 process = 1 task All processes = 1 task
Structure of a task
Task state Segment selectors CS, DS, ES, FS, GS, SS General registers EAX-ESP Flags EFLAGS Instruction pointer EIP Control register CR3
Private paging virtual address space The state of TR Selector in LDTR The I/O map Software interrupt redirection map (Pentium+) Stack pointers to the 0-2 privilege level stacks Link to previously executed task
Involved registers and data structures
Task State Segment TSS
TSS descriptor Task gate descriptor Task register TR The flag NT in EFLAGS
NT = Nested Task
TSS structure
Fields in the TSS 32b – I
Fields in the TSS 32b – II
Fields in the TSS 32b – III
TSS descriptor 32b
Task gate descriptor
Using task gate
Task register
Holds a selector to a TSS descriptor
Hidden part
Current task
Task switching Explicit switch
Explicit task switching as a subprogram using CALL Explicit task switching using JMP As a target is either TSS descriptor or task gate descriptor
Checking EPL ≤ DPL Implicit switch
Implicit switch (CPU makes it during some operation) for interrupt or exception handling A target task is provided by task gate descriptor in the IDT
Return from a task using IRET with pre-set NT in EFLAGS Return from „subprogram“ A target task taken from the LINK field of the current TSS
Task switching – mechanism Obtaining a target TSS Checking EPL ≤ DPL The target task is present and its
length≥67h The target task is available for
jumps or busy for return Paging in current, target TSSs and
all used descriptors Clearing B in the old descriptor for
JMP and IRET, leaving original B (=1) for CALL and IRQ
Clearing NT executing IRET
Storing current state into current TSS
Setting NT in new EFLAGS executing CALL or during IRQ, keeping the NT value from new EFLAGS executing JMP or IRET
Setting B in the new descriptor for JMP, CALL, IRQ, leaving original B for IRET
Loading TR with new TSS descriptor
Loading a new state from TSS Loading new segment
descriptors Executing the new task
Task linking
Only when a task is switched using CALL or an interrupt/exception handling using a task gate
It is not possible to make a recursion
Task management in long mode
Task switching not available All attempts cause #GP
64-bit TSS must exist RSPn – stacks for privilege levels 0-2 ISTn – Interrupt Stack Table I/O map
Fields in the TSS 64b – I
Fields in the TSS 64b – II
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